The present invention relates to an audio level dynamic range compression circuit, and in particular to a compression circuit for use in a motor vehicle.
Audio sound systems in motor vehicles have been greatly improved in recent years, for example by the provision of additional speakers and use of higher quality audio components, particularly compact disc (CD) players. These improvements have, however, increased the need to deal with the background noise generated by the vehicle's engine, and by wind and road noise. Compact disc players have a greater available dynamic range, up to 96 dB, compared with compact cassettes, or AM or FM radio. Quiet sections of music may become inaudible in the presence of noise inside a motor vehicle.
One approach to deal with this problem, an example of which described in patent document JP 55151807-A, is to measure the noise level inside the vehicle using a microphone in the vehicle and to boost the gain of a variable gain amplifier according to the level of the noise. Another approach, described in patent document JP 06303063-A, is to use a speed sensor and increase the gain at higher speeds. Whilst these systems directly address the problem of a vehicle noise, both loud and quiet sections of audio become amplified in the presence of background noise, even though loud sections may not need amplifying to be audible.
Other types of systems have therefore been proposed which measure the audio level and then boost the level of quiet sections and/or decease the level of loud sections. This, however, leads to the problem that if the audio level is changing, the gain may not be adjusted quickly enough, or may be changed too quickly, which may be irritating or distracting for a listener, or even cause audible distortion. In order to overcome this, it is possible to use delay circuitry, such as a digital memory, so that the gain is adjusted in parallel with changes in audio level; one example of this type of compression circuitry is disclosed in patent document JP 06204770-A.
Although these systems may provide solutions to the problems associated with noise in a motor vehicle, in all of these examples there is a need for additional components, such as microphones or digital memories, and associated circuitry which adds to the cost of the audio compression system. It is therefore an object of the present invention to provide a system for compressing the dynamic range of an audio signal which uses, as far as possible, the components associated with a conventional motor vehicle sound system not having audio compression capability.